The present invention relates to a magnetic recording medium applicable to a hard disk device or the like for recording or reproducing information by the use of a magnetic head.
A magnetic recording device, such as a hard disk device, can record and reproduce information on a magnetic recording medium by means of a magnetic head. In a device of the type described, the magnetic head is brought into frictional contact with the recording medium when the device is activated or stopped. On the other hand, the magnetic head moves along the surface of the recording medium after the activation of the device and is kept to float just above that surface. This method has been mainly adapted to record and reproduce information with the magnetic head on the magnetic recording medium and will be referred to as a contact start stop (CSS) method.
The device using the above-mentioned CSS method inevitably has, in principle, tribological problems. Specifically, a head slider is completely brought into contact with the recording medium when the device is activated or stopped. In this event, the head slider is stuck to the recording medium because both the slider and the medium have surfaces subjected to mirror finish, which causes stiction to occur between the slider and the medium. The stiction may interrupt or hamper the rotation of a spindle on which the recording medium is loaded and often makes the operation of the device itself be difficult.
Another problem results from frictional force which occurs between the head slider and the recording medium kept in frictional contact with each other. This frictional force may give rise to abrasion and damages of the head slider and/or the recording medium.
Once such abrasion is advanced to a magnetic layer due to the friction, a so-called head crash phenomenon takes place and brings about destruction of the information recorded on the magnetic layer.
Tremendous studies and attempts have been made to overcome the above-mentioned problems of stiction and head crash. Specifically, an idea is to reduce a contact area on a head disk interface (HDI) between the head slider and the recording medium and to decrease stiction between the head slider and the recording medium. To this end, a mechanical texture is formed on a nonmagnetic base by using a lapping tape technique.
Alternatively, as disclosed in Japanese Unexamined Patent Publication No. 3-113823, a glass base is subjected to a chemical treatment (namely, chemical etching) to form desired irregularities on a surface of a base.
At any rate, an underlying layer and a magnetic layer are successively stacked on the base which has the irregularities on the base surface. Furthermore, a protection film is then stacked on the magnetic layer. This protection film serves to prevent the magnetic layer from being directly slidably contacted with the head slider, thereby to improve the abrasion resistance of the recording medium. For this purpose, the protection layer usually comprises carbon, metal oxide, carbide and nitride. A liquid lubricant, such as perfluoro polyether (PFPE) may be coated on the protection layer.
Requirements have been made to reduce a distance (hereinafter referred to as an effective floating height) between a magnetic sensor provided on the head slider and the upper surface of the magnetic layer of the medium, in order to comply with recent demands for a higher recording density of a magnetic recording device. To this end, a floating height must be kept low on the head slider side along the surface of the recording medium. On the side of the recording medium, it is required to reduce a height of the mechanical texture and, as a result, to diminish variations of the effective floating height on the whole of the magnetic recording medium and to achieve stable recording and reproduction of information. In addition, it may be necessary in some applications to reduce the thickness of the protection film deposited on the magnetic layer.
The above-mentioned method of forming the mechanical texture on the base is, however, liable to widely vary the heights of the irregularities on the magnetic recording medium. This makes it difficult to keep the effective floating height constant. Under the circumstances, when the floating height of the head slider itself is lowered, the head slider is abruptly contacted with the irregularities accidentally appearing on the magnetic recording medium. The floating stability is thus no longer achieved, which causes a problem of unstable recording/reproducing characteristics to take place. In addition, this results in a problem that damages are apt to be spread from accidental contact portions. These observations have taught that the recording medium should exhibit the unevenness which has protrusions of a uniform height and which has a small magnitude for reducing the effective floating height, to achieve a higher recording density of the magnetic recording device.
The surface irregularities with the protrusions of a uniform height have not brought into a complete solution to the conventional problems, although advantages have been gained in view of increasing the recording density. More specifically, another technical challenge related to stiction occurs in dependency on various environmental conditions of a magnetic recording device.
It is a recent trend that down-sizing computer systems makes it possible to use the computer systems at a wide variety of sites where computers are used. In such sites, there are included not only usual offices which are well air-conditioned but also houses or construction spots where temperature and humidity vary widely in an uncontrolled manner. Accordingly, a strong demand has been made for a magnetic recording device which has good weather resistance and which permits stable operation without depending on the environmental conditions.
Among others, humidity is one of the most important factors to satisfy the above-mentioned demand. Specifically, the amount of moisture adsorbed in the head slider and the medium becomes significant and directly controls the above-mentioned "stiction" between the head slider and the recording medium, as disclosed in IEEE Trans. Magn., Vol. 26, 2487 (1990). The stiction in a high humidity condition is very stronger in comparison with that in a condition at ordinary temperature and humidity. In the worst case, the spindle for mounting the magnetic recording medium is completely put into an inactive state.
It is well known by those skilled in the art to form irregularities on the surface of the substrate by means of texturing, as mentioned above, to mitigate the stiction in a high humidity condition. For example, Japanese Unexamined Patent Publication No. 4-89616 discloses a method of forming protrusions on the surface of the substrate to provide the protrusions of a uniform height.
Alternatively, Japanese Unexamined Patent Publication No. 2-214014 discloses a texturing technique of forming surface protrusions that are dispersed comparatively at random.
The former technique can reduce a contact pressure imposed on a single protrusion at the HDI and can improve the head crash resistance. However, water droplets or vapor may easily be coagulated in a high humidity environment, which facilitates occurrence of the stiction.
On the other hand, the latter technique is more effective to prevent occurrence of the stiction. However, only the small number of protrusions can be brought into contact with the head slider, which results in a local increase of the surface pressure imposed on these protrusions. The local increase of the surface pressure may introduce a local damage on the magnetic recording medium. In addition, the local damage may be spread from a portion at which the surface pressure is increased.
As mentioned above, conflicts take place between a reduction of the stiction and an increase of the recording density, when both of them are accomplished by reducing the surface roughness of the recording medium. Accordingly, a strong requirement is directed to a magnetic recording medium that enables a small floating height of the head slider and a reduction of the stiction even in a high humidity condition.